1. Field of the Invention
This invention relates generally to outdoor lighting systems and more particularly to a power source and control system suitable for supplying power to present or next generation outdoor lighting fixtures.
2. Description of the Related Art
Outdoor lighting is popular for security, aesthetic, safety, and other reasons. Energy efficient power sources are desirable, and required by law for certain applications in certain areas.
Until recently, outdoor lighting fixtures housed, almost exclusively, 12V incandescent light bulbs. These bulbs are not very efficient because they generate significant heat. Aside from the in-efficiency issue, this heat needed to be kept in mind when designing the fixture itself, particularly those fixtures made out of plastic.
There are other well-known problems with incandescent bulbs. They often have a short life in outdoor landscape lighting systems and therefore need frequent replacement. Incandescent bulbs can only achieve different colors using shaded lenses. They have a limited “linear” range for intensity changes. This means that they can be dimmed only over a narrow range without big losses of efficiency.
Presently, the typical outdoor lighting power supply consists of a step-down trans-former used to generate 12VAC from 120VAC. Typically the transformer also has a timer either built into its enclosure, or next to it. The timer is used to switch power to the transformer ON and OFF to control all of the lights simultaneously. These devices are popular because they provide a safe, low voltage output that is galvanically isolated from the main 120VAC input line. This allows the simple unprotected burial of the output wiring from these transformers at no particular depth without creating a safety hazard. If, on the other hand, the 120VAC power was to be routed to each load, it would have to buried at a specific depth and in most cases, run through some sort of approved conduit. While local codes may vary, this is generally the case.
While fairly safe, conventional outdoor lighting power supplies are bulky, heavy, and not very efficient. The efficiency of a transformer is simply (output power)/(input power). All transformers have losses resulting in efficiencies less than 100%. The “lost” power is typically dissipated as heat in the transformer. The type and size of transformers used in outdoor lighting applications, which typically fall in the 100 W to 1000 W range, have full load efficiencies from 75%-85%. However, as the load varies, this efficiency decreases greatly. For instance, a 600 W transformer with no load connected may draw as much as 5 or 10 W from the 120VAC main. The efficiency is actually 0% for this no-load scenario and the entire 5-10 W is “wasted” power. Also, a transformer does a poor job of regulating its output voltage. Output voltage is proportional to input voltage, and also affected greatly by load. For instance, a 600 W 12VAC trans-former may put out 12VAC, but only when it is loaded to 600 W. If it is only loaded to 300 W, its output may be 15VAC or more, which can shorten the life of incandescent bulbs. This makes it problematic for the end user to “oversize” the transformer to account for plans to later add more lighting powered by the same system.
New devices are now becoming available for emitting light such as high intensity Light Emitting Diodes (LEDs). Some of these device last longer, are more versatile and are more efficient than the typical incandescent bulb. But existing power supplies and lighting control systems are primarily adapted to powering incandescent lights and generally unsuitable for fully exploiting the many performance advantages of these newer light sources over incandescent bulbs.